Rolling bearings are used in transportation machinery and various other industrial machines to support their various parts so as to be rotatable relative to each other. A rolling bearing includes an outer race formed with a raceway, an inner race formed with a raceway, and rolling elements disposed between the raceways of the inner and outer races, supporting the inner and outer races so as to be rotatable about the axis of the bearing.
The inner race has a bore through which a shaft can be inserted so as to be fixed to the inner race. The outer race can be fixedly fitted in a housing.
Typically, the outer race 1 and the housing H, as well as the inner race 2 and the shaft Ax, are fitted together by clearance fit or interference fit such the outer race and the housing, as well as the inner race and the shaft, do not rotate relative to each other.
In large motor vehicles such as heavyweight dump trucks, tapered roller bearings as shown in FIGS. 19 and 20 are frequently used (see e.g. JP Patent Publication 2009-204016A) because tapered roller bearings are large in load capacity and high in rigidity.
In a rolling bearing used in such a heavyweight vehicle, the outer race 1 and the housing H, and/or the inner race 2 and the shaft Ax may creep, i.e. slightly rotate relative to each other about the bearing axis, when the rolling elements 3 move and the load distribution changes.
Such creeping occurs e.g. when fitting surface pressures from the rolling elements 3 to the inner and outer races change as the rolling bearing 10 rotates, thus causing arbitrary and random deformation of the inner and outer races.
For example, the portions of the outer and inner races 1 and 2 that are in contact with a rolling element 3 located right under the axis of the rolling bearing 10 tend to be deformed in the circumferential direction more markedly than the other portions of the inner and outer races. The further this rolling element 3 moves circumferentially from the position right under the bearing axis, the lower the degree of deformation of the portions of the inner and outer races that are in contact with this rolling element 3. Thus, any portion of each of the inner and outer races is deformed alternately to a larger degree and a smaller degree, so that as a whole, the outer and inner races 1 and 2 are deformed in a wavy pattern in the circumferential direction. This causes creeping between the outer race 1 and the housing H and between the inner race 2 and the shaft Ax. The higher the radial load applied to the rolling bearing 10, the higher this tendency.
If creeping occurs, the contact portions between the outer race 1 and the housing H and between the inner race 2 and the shaft Ax tend to become worn, producing noise and vibration, or causing seizure on the contact surfaces.
JP Patent Publication 2001-1710A proposes an arrangement for reducing creeping, in which in order to reduce creeping between the inner race of a bearing and a hub ring fitted in the inner race, an end of the hub ring is crimped such that a material forming the hub ring is received in grooves formed in the inner periphery of the inner race at its end, thereby rotationally fixing together the hub ring and the inner race.
JP Patent Publication 2010-25155A proposes to manufacture a bearing by an ordinary method, but such that in order to reduce creeping, the inner and outer races both have larger wall thicknesses and thus are more rigid than conventional inner and outer races.
By increasing the wall thicknesses of the inner and outer bearing races, it is necessary to reduce the roller diameter if the number of the rollers is unchanged. This reduces the load rating of the bearing because the load rating is an increasing function of the roller diameter as well as of the number of the rollers. But in JP Patent Publication 2010-25155A, it is stated that even when the wall thicknesses of the inner and outer bearing races are increased, it is possible to increase the load rating by increasing the number of the rollers even though the roller diameter has to be reduced.
Press release “Developing creep-free bearings” (Public Relations Department, NSK Ltd., Jun. 27, 2007; http:jp.nsk.com/company/presslounge/news/2007/press070627.html) proposes, in order to minimize creeping, to optimize the dimensions of an O-ring and a groove formed in the radially outer surface of the outer race in which the O-ring is received, as well as the elasticity and repulsive force of the O-ring.
In NSK TECHNICAL JOURNAL No. 680 (2006) pages 13 to 18 “Mechanism of creeping of bearing outer race”, the results of experiments and numerical analyses are reported regarding creeping of the stationary bearing ring of a bearing in the same direction as the rotational direction of the bearing.
In this document, regarding issues related to the amount of creeping, it is reported that the smaller the ratio of the outer race wall thickness to the rolling element diameter, the higher the creep tendency and that the higher the load ratio (Fr/Cr), the higher the creep tendency too. It is further mentioned that the smaller the number of the rollers, the higher the creep tendency.
In NSK TECHNICAL JOURNAL No. 680 (2006) pages 13 to 18 “Mechanism of creeping of bearing outer race”, the mechanism of creeping is explained as circumferential stretching and shrinkage of the bearing rings, i.e. as the behavior of longitudinal waves. Finite element method (FEM) analysis regarding the amount of creeping is performed by simple two-dimensional calculation.
While the arrangements disclosed in JP Patent Publication 2009-204016A; JP Patent Publication 2001-1710A; JP Patent Publication 2010-25155A and Press release “Developing creep-free bearings” (Public Relations Department, NSK Ltd., Jun. 27, 2007; http:jp.nsk.com/company/presslounge/news/2007/press070627.html) are effective to reduce creeping to some extent under certain conditions, but may not be effective to reduce creeping of bearings to which large radial loads are applied.
In NSK TECHNICAL JOURNAL No. 680 (2006) pages 13 to 18 “Mechanism of creeping of bearing outer race”, a technique concerning certain tendencies regarding creeping is described. But data showing these relations are nothing but qualitative description showing the relationship between relative ratios. Thus, this reference discloses none of inflexion points of a graph related to creeping and clear reference values concerning creeping.